1. Field of the Invention
The present invention relates to a method of manufacturing semiconductor lasers and structure thereof. More particularly, this invention relates to a method of manufacturing semiconductor lasers in which a patterned-substrate is used.
Present-day semiconductor lasers are required to have lower oscillation threshold current, higher efficiency and higher power output. Semiconductor lasers utilizing compound semiconductor material such as GaInP and AlGaInP group can emit visible lasing light of 0.6 .mu.m wavelength and are useful as a light source in POS (Point of Sales) terminals, optical disk apparatus, and laser printers.
2. Description of the Related Art
A semiconductor laser as described above, comprising an AlGaInP-GaInP-AlGaInP double-heterostructure is well known.
A typical prior art semiconductor laser of an AlGaInP-GaInP-AlGaInP double-heterostructure type is disclosed in Japanese Unexamined Patent Publication Tokkai-SHO 62-200786. A cross sectional view of this type is shown in FIG. 1. On an n-GaAs substrate 21, a double-heterostructure of an n-AlGaInP lower clad layer 22, a GaInP active layer 23 and a p-AlGaInP upper clad layer 24 which includes a mesa-stripe 24a are formed in this order. Current-blocking layers 26 of n-GaAs are formed on the upper clad layer 24 so as to bury the mesa-stripe 24a, and a p-GaInP contact layer 25 and a p-GaAs contact layer 27 are formed on the mesa-stripe 24a, and a p-GaAs contact layer 28 covers the entire underlying structure.
In the above structure, the mesa-stripe 24a of p-AlGaInP upper clad layer 24 has a function of guiding the lasing light; however, the current-blocking layers 26 on both (i.e., the opposite) sides thereof work as a loss-guide. The semiconductor laser of this type, therefore, is undesirable because a high efficiency and high power output can not be expected.
As another type of semiconductor lasers of the prior art, a buried heterostructure type is known, in which a AlGaInP-GaInP-AlGaInP double heterostructure is etched until a stripe-shaped ridge is formed, and the removed portions on both sides of the stripe-shaped ridge are buried with current-blocking layers. However, this type also presents difficulties in fabrication steps.
To solve the above problems, a semiconductor laser structure of the patterned-substrate type shown in FIG. 2 is proposed by Japanese Patent Application Tokugan-Hei 3-92341 having priority date of Jun. 20, 1990. (The same patent was filed as U.S. patent application Ser. No. 691620 dated Apr. 25, 1991, and also filed as European Patent Application No. 91-303783.4). In the application, an AlGaInP-GaInP-AlGaInP double heterostructure is formed on the patterned-substrate having a mesa stripe 31a of a p-GaAs substrate 31 and n-GaAs current blocking layers 32. On the patterned-substrate, a p-AlGaInP spike reduction layer 33, a p-AlGaInP lower clad layer 34, a GaInP active layer 35, an n-AlGaInP upper clad layer 36 and an n-GaAs contact layer 37 are formed in this order. Each of these stacked layers forming a heterostructure has downward inclination on both sides of the mesa-stripe 31a, thereby forming a gradual slope which continues to a comparatively flat layer. The structure does not include a loss-guide structure, and control of transverse lasing mode is attained by a bent shape of the heterostructure.
In conjunction with the laser structure of the above patterned-substrate type, Japanese Unexamined Patent Publication Tokkai-Hei 4-133315 discloses a method of improving a shape of the patterned-substrate having the GaAs substrate 31 with the mesa stripe 31a and the current-blocking layers 32 on the substrate 31 shown in FIG. 2. A SiO.sub.2 layer is first deposited on a flat original GaAs substrate, and next, the SiO.sub.2 layer is patterned in a stripe shape and, thereafter, the substrate is subjected to a wet-etching process using, for example, a mixed solution of H.sub.2 SO.sub.4, H.sub.2 O.sub.2 and H.sub.2 O. Next, the n-GaAs current blocking layers 32 are deposited thereon by a MOVPE (Metal Organic Vapor Phase Epitaxy) method.
The semiconductor lasers of the patterned-substrate type have capabilities of high efficiency and high power output, however, a current confining function is not satisfactory because the vertical lasing current through the stacked layers is apt to spread outwardly in the transverse direction. Further, the heterostructure above the mesa-stripe 31a (lasing portion) is not formed of ideal flat layers but it instead comprises small undulation which causes splitting of the transverse lasing mode with the result of deteriorating a far field pattern of the lasing light.